Various types of tests related to patient diagnosis and therapy can be performed by analysis of patient fluid samples. For the analysis, such patient samples are typically placed in sample vials, extracted from the vials, combined with various reagents in special reaction cuvettes, incubated, and analyzed. In typical clinical chemistry and some immunochemical analyses, one or more assay reagents are added to a liquid sample, and the sample-reagent combination is mixed and incubated within a reaction cuvette. Photometric measurements using a beam of light illuminating the sample-reagent combinations in such reaction cuvettes are made from which an amount of analyte may be determined using known techniques. Examples of such photometric measurements comprise turbidimetric, fluorometric and absorption measurements or the like.
There is an ever growing need to increase the throughput of said analyses, to make them faster, less expensive, and simpler to perform while at least maintaining, if not increasing, precision and reliability. In order to achieve this goal, substantial effort has been devoted to miniaturization, parallelization, and integration of various process steps, e.g., by processing several cuvettes at a time in a fully automated analyzer comprising pipetting units, reagents and an optical system. Particularly there is a tendency to decrease reaction volumes thus minimizing consumption of samples and reagents, reducing costs and waste volumes.
Conventional cuvettes are optimized for mixing and for the photometric measurement of liquid volumes in the order of 100 μL or more. When trying to use the same cuvettes for smaller volumes, problems arise, such as inefficient mixing and adverse capillary effects. This in turn results in more difficult positioning during optical detection. In particular, as the liquid level becomes lower this makes the photometric measurement unreliable.
When trying to use smaller cuvettes instead, the capillary effects become even more severe than for larger cuvettes due to an increased surface to volume ratio. Thus the measurement is even less reliable.
In case of liquid-surface interfaces characterized by large contact angles, another and rather incontrollable effect may be observed, which is tilting of the liquid surface. For small volumes this may have a significant negative impact on the reliability of the photometric measurement.